Treatment of steel



Patented Apr. 15, 1941 UNITED STATES PATENT OFFICE mum-r or srsm.

Vincent '1. Malcolm, Indian Orchard, m,

lignorio'lhechamnanvalvemanuf u 00., Indian Orchard, Munch uletts Mara, a corporation of No Drawing. Application June 21, 1940,

. Serial No. 341,703

4 Claims. (Cl. 148-8) austenitic steels that when parts thereof have been in contact with other steels or other parts thereof and there is any relative movement, the austenitic steel has a tendency to score, scratch and seize. For example, when a valve or seats of a valve are made from ordinary austenitic steel, the action of one part on another results in scratching, scoring, or galling to such an'extent that the part or parts are soon unfit for service, notwithstanding the fact that austenitic steel has otherwise such desirable characteristics as ability to resist corrosion, withstand high temperatures, etc.

To put it another way, a major disadvantage of the austenitic steels has been its characteristic of a brittle hard surface after certain operations in forming the work. Due to this peculiar conditicnknown as work-hardening, considerable difllculty has been experienced because of a brittle condition in numerous locations throughout the finished piece, which subsequently causes premature local failures when the pieces or parts are used in various service conditions.

Steels of the type to which the invention relates contains chromium and nickel in the relative percentages of about 18 and 8, with from say .02 to 25% carbon, or wherein the chromium and nickelmay be 25 and 12% respectively, or even in the reverse order.

I have found that the surface failure which I have mentioned is an objectionable characteristic of ordinary austenitic steels unlike usual surface failures or attacks in that it is more or less localized and tends to progress horizontally along the body of the steel piece. That is to say, the condition of local attack occurs at various points on the piece's surface and progresses in a generally horizontal direction therealong.

This horizontal attack is particularly noticeable when the steel is operated under certain conditions and those skilled'in the art are generally agreed that these points of attack or initiation of local failures are attributed to the condition of work hardening which ultimately results in excessive straining and subsequent cracking of the steel at localized points. The factors involved in the solution of this problem are so complicated and baiiling that investigators have been unable to come to any agreement either as to the importance of the conditions encountered or as to how toapproach theovercoming of the disadvantages commonly known.

After having thoroughly investigated the whole problem of the susceptibility of austenitic steels to work hardening, both from the mechanical and metallurgical aspects, I have 'conceived a means of solving the problem.

As a special feature of the invention, I have conceived a special bath having the unique characteristics and ability of dissolving off from the surface of the steel any localized brittle sections resulting or developing from previous machining or other operations. The preferred composition of this bath will presently be described but it may be here stated that it consists generally of a combination of tin chloride, nitric acid, and water.

In United States Letters Patent #2,131,710 issued to me on Septemberj'l, 1938, I disclosed, as a means of eliminating the objectionable surface brittleness in austenitic steels, the treating thereof with disassociated ammonia gas in such a way that a considerable amount of occluded, adsorbed or absorbed gases were removed, and non-metallic inclusions and oxides were broken down and reduced from the surface. It is to be understood that my new method may or may not include this Just-mentioned treatment after the immersing of the steel in the new bath.

The preferred. proportions of the ingredients of the bath are substantially as follows:

- Per cent Nitric acid (spec. gr. l.42) 20 Tin chloride 8 Water 72 oughly washed in hot water.

I Thereafter the work or pieces of steel are preferably scrubbed and thor- I have not only found that the bath serves to positively remove the work hardening from the metal but that it imparts to it an improved lustre or brightness. It results in the removal of various inhomogeneities from the metal and elimination of surface imperfections and brittleness.

It will be noted that prior to the immersion in the bath, the work is machined. After the bath treatment it is not intended that the steel be machined but, as stated, it may then be further treated by subjecting it to the action of disassociated ammonia gas.

This latter treatment preferably consists of placing the work in a closed container which, in turn, is placed within a furnace heated to a preferred temperature within the range of from 900 to 1700 F. Into the furnace are introduced the products or constituents of ammonia gas in such a way that the same contact the work, preferably after being passed through scrubbing, drying, cracking and catalyzing units, all of which apparatus may be similar to that disclosed in United States Letters Patent #2,131,709 issued to me on September 27, 1938.

According to my United States Letters Patent #2,l31,710 referred to further above, the resultant ammonia gas products are led to the work and the heat is maintained within the temperature range stated for one-half hour or more. Then the flow of the gas products is cut off and ammonia gas is passed through an ionizing unit and led into the compartment containing the steel.

The temperature then employed is preferably from 850 F. to 1400 F. and the time of treatment varies, depending upon the work being operated upon, from 4 to as much as 50 hours.

It will be appreciated that I have above disclosed a unique treatment for austenitic steels whereby I not only refrain from destroying or minimizing their desirable characteristics of corrosion-resistance, etc., but render their surfaces truly hardened and wear resisting.

Manifestly the specific details of construction may be considerably varied from those herein shown and described without involving any departure from the principle of the invention or sacrificing any of the advantages inherent therein. While I have described my invention in this specification in great detail and particularly with respect to the present preferred form thereof, it is not desired to be limited thereto since many changes and modifications may be made therein without departing from the spirit and scope of the invention.

The invention may be embodied in other speciflc forms without departing from the essential characteristics thereof. Hence, the present embodiments are therefore to be considered in all respects merely as being illustrative and not as being restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all modifications and variations as fall within the meaning and purview and range of equivalency of the appended claims are therefore intended to be embraced therein.

What it is desired to claim and secure by Letters Patent of the United States is:-

1. The method of treating austenitic steel consisting of, immersing the same in a solution including tin chloride and nitric acid, and heatilng) the same to a temperature of approximately 2. A solution for the treatment of austenitic steels to provide the same with non-work-hardening surfaces which consists of, approximately 72% water,1approximately 20% nitric acid, and approximately 8% tin ,chloride.

3. The method of treating austenitic steels to provide the same with non-work-hardning surfaces which consists in, immersingthe steel in a solution includingtin chloride andnitric acid,

V and then heating the same to a temperature of approximately 4. The method of treating austenitic steels to provide the same with non-work-hardening surfaces which consists in, immersing the, steel in a solution including tin chloride and nitric acid and heating the same to a teinperature o'f approximately 180 F. and then subjecting the same to the action of the activated constituents of ammonia gas.

VINCENT T. MALCOLM. 

